1. Field
Embodiments of the present invention relate to image sensors and, in particular, to metal reflectors for image sensors.
2. Discussion of Related Art
In general, conventional image sensors perform well to generate images. A typical image sensor may be fabricated from a complementary metal oxide semiconductor (CMOS) technology. Charge couple device (CCD) technology is also suitable. Such image sensors include an array of pixels disposed on a substrate, each pixel including a photosensitive element, such as a photodiode, for example, a filter disposed on the photosensitive element, and a microlens disposed on the filter.
The typical image sensor operates as follows. Light such as visible light is incident on the micro-lens. The micro-lens focuses the light to the photosensitive element through the light filter. The photosensitive element converts the light into an electrical signal proportional to the intensity of the light detected. The photosensitive element and a floating diffusion region may be coupled together when an associated transfer transistor is turned on to transfer the charge from the photosensitive element to the floating diffusion. The electrical signals may be coupled to amplification and readout circuitry such as a CMOS transistor (not shown), for example, to generate an image based on the captured light. There may be one or more layers of metal, polysilicon, etc., disposed on the substrate as well.
Some image sensors may use front side illumination (FSI) in which the layers of metal, polysilicon, diffusions, etc., may be disposed on the same side of the substrate as the micro-lenses and the photosensitive elements. Other image sensors may use back side illumination (BSI) in which the metal layers are on one side of the substrate and the photosensitive elements are on another side of the substrate. In either case, because of the position of the pixel in the image sensor light may have to be focused by the micro-lenses from different angles down onto the photosensitive elements.
Unfortunately, as the angles become larger, some of the light is not focused onto the photosensitive elements, which causes light to be lost and pixel response to decrease. The chief ray is the ray which passes through the center of a camera entrance pupil, and light near the center of the camera entrance pupil enters the pixel. The angle of the chief ray is commonly called the chief ray angle (CRA). At large CRAs, the pixel response drops below a certain percentage (e.g., 80%) of its 0° angle response where the pixel is perpendicular to incident light. The light near the axis of a camera lens enters the pixel with angles near 0°; however as the light moves further from the axis, the angles increase. As a result, there can be crosstalk between the pixels located away from the center of the array. Crosstalk creates noise in the image sensor.